Allow FormData to be passed as input to AI bindings

```
Models like bfl/flux-2-dev accept FormData as inputs, where the user
can pass multiple images as style references. This PR allows AI bindings
to pass FormData as-is to the model.
```

Author: thatsKevinJain

src/cloudflare/internal/ai-api.ts

@@ -42,7 +42,7 @@ export type AiOptions = {
 };
 
 export type AiInputReadableStream = {
-  body: ReadableStream;
+  body: ReadableStream | FormData;
   contentType: string;
 };
 
@@ -87,7 +87,6 @@ export class AiInternalError extends Error {
   }
 }
 
-// TODO: merge this function with the one with images-api.ts
 function isReadableStream(obj: unknown): obj is ReadableStream {
   return !!(
     obj &&
@@ -97,6 +96,17 @@ function isReadableStream(obj: unknown): obj is ReadableStream {
   );
 }
 
+function isFormData(obj: unknown): obj is FormData {
+  return !!(
+    obj &&
+    typeof obj === 'object' &&
+    'append' in obj &&
+    typeof obj.append === 'function' &&
+    'entries' in obj &&
+    typeof obj.entries === 'function'
+  );
+}
+
 /**
  * Find keys in inputs that have a ReadableStream
  * */
@@ -111,9 +121,9 @@ function findReadableStreamKeys(
       value &&
       typeof value === 'object' &&
       'body' in value &&
-      isReadableStream(value.body);
+      (isReadableStream(value.body) || isFormData(value.body));
 
-    if (hasReadableStreamBody || isReadableStream(value)) {
+    if (hasReadableStreamBody || isReadableStream(value) || isFormData(value)) {
       readableStreamKeys.push(key);
     }
   }

src/cloudflare/internal/test/ai/ai-api-test.js

@@ -214,6 +214,25 @@ export const tests = {
       );
     }
 
+    {
+      // Test form data input
+      const form = new FormData();
+      form.append('prompt', 'cat');
+      const resp = await env.ai.run('formDataInputs', {
+        audio: {
+          body: form,
+          contentType: 'multipart/form-data',
+        },
+      });
+
+      assert.deepStrictEqual(resp, {
+        inputs: {},
+        options: { userInputs: '{}', version: '3' },
+        requestUrl:
+          'https://workers-binding.ai/run?version=3&userInputs=%7B%7D',
+      });
+    }
+
     {
       // Test gateway option
       const resp = await env.ai.run(

src/cloudflare/internal/test/ai/ai-mock.js

@@ -97,6 +97,19 @@ export default {
       );
     }
 
+    if (modelName === 'formDataInputs') {
+      return Response.json(
+        {
+          inputs: {},
+          options: { ...data.options },
+          requestUrl: request.url,
+        },
+        {
+          headers: respHeaders,
+        }
+      );
+    }
+
     if (modelName === 'inputErrorModel') {
       return Response.json(
         {